The present invention relates to the transmission of data over telecommunications networks.
Recent advances have made possible the design of voiceband modems capable of achieving data rates as high as 56 kilobits per second (kbps). At the heart of this technology is the realization that one can completely eliminate the quantization noise introduced by PCM vocoders within the telephone networkxe2x80x94which acts as a source of impairment in the overall channelxe2x80x94by encoding the data bits to be transmitted using the codes which represent the vocoders"" quantization levels and delivering those codes to the network in their digital form. By thus matching the amplitudes of the transmitted signalxe2x80x94actually represented by an 8-bit wordxe2x80x94to the predefined quantization levels of the vocoder, the receiving vocoder""s analog output amplitude is made to be an exact, rather than an approximated, representation of the input amplitude. In essence, this approach implements a modulated signaling scheme based on a constellation of signal points derived from the quantization levels of the vocoder. Such a constellation is herein referred to as a xe2x80x9cPCM-derived constellation.xe2x80x9d See, for example, my co-pending U.S. Pat. application Ser. No. 08/53,351 U.S. Pat. No. 5,953,376, filed Nov. 25, 1996, hereby incorporated by reference, and the parent application hereof, which describe arrangements using trellis-coded modulation with a PCM-derived constellation in a way which achieves significant coding gain.
A potential problem in the practical implementation of this technology results from the fact that some PCM links, at least in North America, utilize so-called bit robbing in order to provide an in-band signaling channel for signaling and maintenance purposes. In particular, for one out of every six of the 8-bit words representing the various PCM quantization levels, the least significant bit of that word is preempted by the telephone network to implement that in-band channel. In practical effect, this means that there is a 50% chance that a different word than the one that was transmitted will be received at the far-end central office. The mapping between the 8-bit patterns and the various PCM quantization levels is such that when this occurs, the quantization level that is identified by the modified 8-bit word is adjacent to the original quantization level. From the standpoint of voice communications, this substitution of one quantization level for the adjacent one is not noticeable to the listener. However, it has potentially much greater impact for data communication because unless some mechanism is in place to compensate for this effect, there will be a very large, perhaps catastrophic, impact on the bit error rate. Indeed, it is possible that within different PCM links involved in a particular connection, the bit robbing may be applied to different ones of the 8-bit words, thereby making this problem all the worse. In addition, when the modulation scheme implemented in such modems includes, for example, trellis encoding, the problem may become even more severe because unless something is done, error propagation effects may cause erroneous data recovery vis-a-vis not only the data transmitted during bit-robbed intervals but also the data transmitted in non-bit-robbed intervals.
In accordance with the present invention, the bit-robbing phenomenon is accommodated by using the output of a single redundancy encoder (e.g., trellis encoder) to identify signaling constellation subsets for both the bit-robbed and non-bit-robbed intervals, the number of signal points in the subsets being less for the bit-robbed intervals than for the non-bit robbed intervals. The reduced data rate supportable by the smaller subsets used for the non-bit-robbed intervals is consonant with the fact that a bit is robbed and, all other things being equal, the channel can only support a lower data rate during such intervals.
An alternative approach that one might think to employ is to use two separate trellis encoders, one for the bit-robbed signaling intervals and one for the non-bit-robbed signaling intervals. Such an approach, however, will engender significantly decoding delay in the receiver because a particular number of signal points transmitted during the bit-robbed signaling intervalsxe2x80x94equal to the so-called decoding depthxe2x80x94would have to be received before a decoding decision can be made, thus increasing the decoding delay by six-fold, for example, in the case where bit robbing occurs once every six signaling intervals. The fact that the present invention uses a single coder across both bit-robbed and non-bit-robbed intervals, however, advantageously avoids any such decoding delay problem.
Bit robbing is but one example of a phenomenon wherein channel conditions may change in a known, or deterministic way, and the invention is thus potentially more generally applicable to any coded modulation environment in which the channel conditions may so change.